A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A conventional lithographic apparatus includes a base frame which is supported on the ground and a vibration isolated frame often referred to as metro-frame. The metro-frame is substantially isolated from external influences, such as vibrations in the base frame, by the use of vibration isolating mounts, such as air mounts, which support the metro-frame on the base frame. These isolated mounts may be actively controlled to isolate vibrations which are introduced by the base frame and/or by the isolated frame itself. These latter vibrations may for instance be caused by movement of stages or other moving parts arranged on the metro-frame. On the metro-frame the projection system is mounted. As the metro-frame is substantially isolated from vibrations with the vibration isolating mounts, the accuracy of projection is substantially increased.
In conventional lithographic apparatus, a cable connection is provided between the base frame and the metro-frame. Such cable connection has a cable bundle which runs from the base frame to the metro-frame. The cable bundle includes a number of cables which are typically used to provide a connection for electricity, compressed air, or compressed fluid between the base frame and the metro-frame. The electrical connection may for instance be used to energize a certain part on the metro-frame or the electrical connection may be used to transfer a measurement or control signal, for instance between a sensor or actuator arranged on the metro-frame and a central control unit which is mounted on the baseframe.
To avoid that the cable bundle passes on or transfer vibrations from the base frame to the metro-frame, the cable bundle is arranged in a wave-shape or twist so that these vibrations are absorbed to a certain extent by the flexibility of the cable bundle.
However, since in general more and more cables have to be guided from the base-frame to the metro-frame, the flexibility of the cable bundle decreases, and as a consequence vibrations are passed on or transferred more easily. On the other hand, requirements on the vibration isolation in the metro-frame are constantly increasing due to general requirements with respect to overlay and throughput performance of the lithographic apparatus.
In a lithographic apparatus, the stiffness of the wave-shaped cable bundle may come in the same range as the stiffness of the vibration isolating air-mounts which support the metro-frame on the base frame. As a consequence, base frame vibrations may excite the metro-frame, creating larger noise levels in the metro-frame, which results in larger lens vibrations. It will be clear that this is undesirable as it has a direct influence on the accuracy of the projection process, and thus overlay performance of the lithographic apparatus.